The increasing complexity of integrated circuits and the trend to their ever faster switching capabilities has led to an astonishing shrinking of their geometries to submicrometer dimensions. The requirement to keep connection lines between the different circuits as short as possible, has resulted in the development of multilayer circuit packages having a plurality of interconnections between their layers.
These interconnections are usually called "via connections" or "through-connections" because they extend from one plane or level of an integrated circuit package through one or more ceramic or semiconductor substrates to one or more other levels thereof, connecting electrical conductors as dictated by the design of the circuit interconnections.
Typically, the via connections comprise a hole from one surface of a substrate to the other, possibly manufactured "in situ" as the substrate is manufactured, or later drilled into the pre-fabricated body of the substrate, an electrical connector inside the hole and a cover, usually termed stud on both sides of the substrate for contacting the circuit lines thereon. In situ fabrication of the holes may, for example, be done by conventional photolithographic techniques; drilling is usually performed with a highly focused laser beam. The connector inside the via hole may be, for example, an electrically conductive layer of a metal covering the wall of the hole. Such a metal layer can be deposited by way of evaporation or sputtering.
In view of the enormous packaging density of the components making up a modern integrated circuit the via holes have diameters in the micrometer range. And there are many holes on a single layer or module.
While all manufacturers of integrated circuit packages observe the greatest care and cleanliness during fabrication, it may happen that one or more of the fabrication steps have some undesired material inside the via hole or in its neighborhood, so that the deposition of a homogeneous metal layer inside the hole and the proper contacting of the circuit lines on the substrate by the studs are impaired. This could then result in a missing or bad electrical contact and, accordingly, in a defective circuit package. This is true particularly where insulating material on top of, or inside, a via hole covers more than a certain area of the wall thereof.
The fact that the manufacture of an integrated circuit package involves a great number of very sophisticated process steps, causes the package to quickly become an extremely expensive item. Accordingly, defective packages are not easily disposed, but great effort to their repair will be made in most cases. An important aspect in this connection is the inspection of the package at various stages during manufacture.
Presently, the inspection of ceramic and thin film integrated circuit packages is done visually and, accordingly, faces severe difficulties where the checking of the via holes is at issue. On the one hand, it is hardly possible to visually distinguish between the different materials used in modern packages, such as chromium and polyimide, for example, because of their similar optical characteristics. On the other hand, the via holes are so small and so numerous in each integrated circuit board that their visual inspection by operators requires an impossibly long time.
An automatic inspection systems based on a sufficiently large contrast between the materials used in the manufacturing process, in particular between metals and insulators would be an important improvement in the manufacture and testing of integrated circuit packages. Most importantly, such an inspection system would have to provide information on insulating remnants possibly covering more than a negligible part of the surface of a via connection in an unambiguous and fast way.
To this end, the inspection system must rely on a large materials contrast between metals and insulators, allow a short signal integration time to achieve said contrast, and offer the possibility for automatic operation under the control of a computer. Preferably, the system should not require images to be processed, as this takes a lot of computer time and too much storage space in the computer.
The present invention contemplates an automatic inspection system for checking via holes in ceramic and thin film packages, in which the large materials contrast between metals and insulators resulting from light beam induced photoemission is employed. The photoemission technique relies on the known photoelectric effect whereby radiation of sufficiently high frequency impinging on certain substances causes bound electrons to be given off with a maximum velocity proportional to the frequency of the radiation, i.e. proportional to the entire energy of the photons. While any lights source providing essentially monochromatic light of the required frequency will do, a laser device will most probably be the best choice.
Laser testing of integrated circuits is a technique will know in the art. There are several different methods to be distinguished which will be described briefly in the following to make it clear that they can not replace the technique of the present invention.
A straightforward method is to shine laser light onto the surface of the specimen and to monitor the reflected light with a photodetector and associated electronics. Defects on the surface of the specimen will cause the light to be deflected away from the optical axis of the system and, thus, lead to a dimmer than expected reflected wave. This method is, for example, described in CH-A-662 888.
A second method, described, e.g., in EP-A-264 481, which corresponds with U.S. Pat. No. 4,843,329, and EP-A-264 482, which corresponds with U.S. Pat. No. 4,868,492, employs a focused laser beam for irradiating an area of a specimen in order to generate a positive charge at the spot of impact of the beam through photoemission of electrons. The charge will distribute such that all conductive material connected with the spot of impact will assume the same voltage. Flooding the specimen with a second laser beam will now cause photoemission to be detectable from those areas not previously charged.
In contrast, the method of the invention relies on the special features of photoemission with photon energies of about 5 eV where there is a high materials contrast between metals and insulators owing to the large differences in their work functions, which are on the order of 4 eV for the metals used in integrated circuits, and about 7 eV and above for insulators.